1. Field of the Invention
The present invention relates to a projection system, in particular for a microlithographic projection exposure apparatus.
2. Description of Related Art
Microlithography is used in the fabrication of microstructured components like integrated circuits, LCD's and other microstructured devices. The microlithographic process is performed in a so-called projection exposure apparatus comprising an illumination system and a projection system. The image of an illuminated mask (or reticle) is projected onto a resist-covered substrate, typically a silicon wafer bearing one or more light-sensitive layers and being provided in the image plane, in order to transfer the circuit pattern onto the light-sensitive layers on the wafer.
Generally, the image field of any optical system such as the projection system of the projection exposure apparatus is curved, and the degree of curvature is determined by the Petzval sum. The correction of the Petzval sum is becoming more and more important in view of the increasing demands to project large object fields with increased resolutions.
Various attempts to avoid curved image fields are known. Their following discussion is not exhaustive and given without prejudicial admission of prior art.
One group of approaches to overcome the problem of curved image fields includes the use of curved substrates and/or masks having a curvature which is the negative of the curvature of the optical system. Reference is exemplarily given to U.S. Pat. No. 5,257,139 (to Higuchi).
A further approach is the use of catadioptric systems, which combine both refracting elements (e.g. lenses) and reflecting elements (e.g. mirrors). While the contributions of positive-powered and negative-powered lenses in an optical system to power, surface curvature and chromatic aberration are opposite to each other, a concave mirror has positive power like a positive-powered lens, but an opposite effect on surface curvature without contributing to chromatic aberrations.
Various design alternatives for catadioptric systems are known in the prior art.
U.S. Pat. No. 5,052,763 (to Singh et al.) discloses an optical system having an input subsystem (e.g. catadioptric system) which has a curved image field being the input object to a second (e.g. dioptric) subsystem the output of which is a flat image field, wherein the first subsystem is designed to compensate for the field curvature of the second subsystem to result in a substantially flat image field. The input subsystem provides an image of an object on an intermediate image surface having a curvature being substantially opposite of the field curvature of the second subsystem.
A different, single-axis projection system is disclosed in WO 01/055767 A3 and U.S. Pat. No. 6,663,350 B2 (to Shafer), said projection system being devoid of planar folding mirrors and being preferably composed, in sequence from the object side towards the image side, of a catadioptric group giving a real intermediate image, a catoptric or catadioptric group giving a virtual image and a dioptric group giving a real image.
Catadioptric designs with an outer-axially used concave field mirror are disclosed in WO 01/51979 A2 (Hudyama, Shafer et al.) and JP 2003-114387 (to Yasuhiro).
U.S. Pat. No. 4,812,028 (to Matsumoto) discloses a reflection-type reduction projection optical system comprising a first and a second subsystem which are combined to set the total Petzval sum to zero. More particular, the radii of curvatures of the first and second optical subsystems are selected such that the corresponding Petzval sums add up to zero, thereby completely correcting the Petzval sum of the entire system.
WO 03/050587 A2 (Shafer et al.) discloses a projection optical system having a catadioptric first objective part, which comprises a beam diverter device and a concave mirror, and a dioptric second objective part. The beam diverter device in the catadioptric first objective part comprises a first fold mirror surface diverting the radiation arriving from the object plane onto the concave mirror of the catadioptric first objective part, and a second fold mirror surface diverting the radiation arriving from the concave mirror to the dioptric second objective part. An intermediate image is created near the second fold mirror surface. In order to enable a design of the dioptric second objective part with relatively small lens dimensions, a positive refractive element is arranged behind the first fold mirror surface between the first fold mirror surface and the concave mirror.
U.S. Pat. No. 5,488,229 (to Elliott and Shafer) as well as U.S. Pat. No. 5,031,976 and U.S. Pat. No. 5,717,518 (to Shafer) disclose two-reflector configurations in a Deep-UV microlithography system, wherein an intermediate image produced by two reflectors (one being concave and one being substantially planar) is flattened by a group of refractive elements. Herein, the intermediate image is created near a central opening (aperture portion) in the concave mirror such that light from the intermediate image passes through the concave reflector to be reflected by the planar reflector onto the concave reflector and finally back to the planar reflector, in order to then pass through the same. A field lens group providing aberration correction is arranged at the position of the intermediate image or displaced relative to the same.
U.S. Pat. No. 6,600,608 B1 (to Shafer et al.), being assigned to the assignee of this invention and incorporated herein by reference, discloses an axially symmetric objective having, between two refractive partial objectives, a catoptric partial objective comprising, for making Petzval- and colour-correction, two opposite concave aspheric mirrors having two negative lenses between them and being provided with central holes, wherein said holes are arranged next to the intermediate images. A further embodiment (FIG. 3 of U.S. Pat. No. 6,600,608 B1) has a purely catoptric partial objective and a purely refractive partial objective, with the mirrors of the catoptric partial objective being purely used for Petzval correction (i.e. correction of the field-curvature) in order to relieve this burden from the refractive partial objective.
WO 2004/019128 (Omura et al.) discloses a combination of a reflection/refraction-type optical system and a configuration of a liquid-immersion optical system, in order to achieve a large image-side numerical aperture and a wide effective image forming area. For this purpose, in a lithographic immersion projection system the light is also passed through a catadioptric anastigmat comprising a concave mirror and at least one negative powered (Schupmann-) lens.